Aerospace, automotive and other industries have been continuously exploring the opportunities to reduce manufacturing cost of various parts, segments, assemblies and sections of end products manufactured. Moreover, these industries have been seeking the opportunities to reduce the weight of the components by replacing conventional materials utilized for manufacturing with other materials such as aluminum, magnesium, composites, etc., which necessitates joining of these parts.
For example, in an automotive industry, a body structural weight can be reduced by replacing conventional steel with aluminum particularly for a closure panel such as roof, deck lid, etc. However, it is very difficult to completely replace steel with aluminum or composite material because of various strength requirements defined by certain regulations and standards such as Federal Motor Vehicle Safety Standards (FMVSS), ECE etc. Therefore combination of advanced high strength steel (AHSS) and aluminum has been looked as a feasible solution for enabling weight reduction of the components and thereby the manufacturing cost. But there are challenges when it comes to joining of these materials.
Conventionally, for spot joining of plates made of aluminum alloy or the like, a resistance spot welding or a mechanical fastening using rivets is employed. The resistance spot welding method have high running cost due to need for a power supply of a large capacity in spot joining, short life duration of welding electrodes due to contamination or wear-out, and a need for utilities such as water and air in addition to a welding power. In the resistance spot welding method, since indentations remain on joined portions, this method is undesirable particularly when applied to outer plates for automobile in addition to creating an uneven geometry; the indentation also deteriorates its external appearance.
In particular case of resistance spot joining of aluminum alloy, as the electrodes significantly wear out after continuous use, its ability to perform continuous spotting is low. When current is conducted, it flows through spots close to a spot to be conductive, that is, current is shunted. Hence, it is impossible to place the spots close to one another. Hence, a required strength is not obtained. Further, the resistance spot welding process is not suitable for welding aluminum to aluminum and aluminum to steel. Therefore, the resistance spot welding was replaced by a friction stir spot welding process.
FIG. 1 illustrates a conventional friction stir spot welding process for combination of Al6061 and steel. As illustrated in FIG. 1, at step (101), upper Al6061 sheet (1) and a bottom steel sheet (2) to be welded are clamped together. Further, the weld cycle is started at step (102), wherein a non-consumable tool (3) made of Polycrystalline Cubin Boron Nitride (PCBN) material is pushed in the direction of the two sheets under a strong compressive force. When a pin section (4) of said tool (3) is in contact of the upper Al6061 sheet (1), the upper Al6061 sheet (1) is plasticized and the material under the pin (4) is softened that facilitates the penetration of the tool at the interface of the two Al6061 sheets.
As illustrated in FIG. 1, at step (103), greater heat is generated as the shoulder (5) section of the tool (3) is in contact with the upper Al6061 sheet (1). The heat is generated due to friction between the contacting surfaces of the upper sheet (1) and the tool (3). As a result of this, the material around the pin (4) is pushed and stirred to form a metallurgical bond joining the two Al6061 sheets and thereby completing the welding process.
Further, as illustrated in FIG. 1, after the completion of the welding process, at step (104), the tool (3) is retracted from the weld region. Upon completion of the welding process, a void is observed at the welded surfaces resulting into unreliable welding.
However, the conventional friction stir spot welding process is not reliable for joining two dissimilar materials with high weld strength. The conventional friction stir spot welding process results in the formation of void in the weld region.
FIG. 2(a) and FIG. 2(b) illustrates weld specimens (201) and (204) formed due to joining of Al6061 and steel material using a conventional friction stir spot welding process. The weld specimens welded are cut perpendicular to top surface of the upper Al 6061 sheet and passing through the center of the weld. As illustrated from FIG. 2(a), a void (202) is formed at the center of the weld. Further, there is lack of bond in the weld region. Further, as can be observed from FIG. 2(a), unbound region (203) can be seen in the weld interface of the two sheets. Such unbound region is formed due to the presence of oxide layers since there is no relative motion between the interface surfaces in the conventional spot welding process.
FIG. 2(b) illustrates a weld specimen (204) formed by the conventional welding process that is subjected to microstructure analysis. As illustrated in FIG. 2(b), a very small contact zone (205) is formed in the welding region of the two welded materials. The weld contact zone in this type is very small and it is only at the inner periphery and at the adjacent interface surface indicated as (205). Due to small contact zone, the weld strength is low and it also leads to unreliability of weld joint.
Therefore, the conventional friction stir spot welding process has following limitations:                Very small contact zone between the adjoining material surfaces.        Ineffective removal of oxides and other contamination from the weld interface (as there is no relative motion between the interface materials)        Lower weld strength        Poor mixing of materials (sheets) used for lap joint welding (as there is no relative motion between them)        Large heat effected zone        The conventional friction spot welding process lack in joining of two non-weldable dissimilar materials.        
In view of the above limitations (except joining of two non weldable dissimilar materials), a refill friction stir spot welding process is employed, In refill friction spot welding the cavity is refilled during the welding process, but it still has the tendency of void formation, as sometimes it fails to refill completely the plasticized material in the cavity resulting in the void at the middle of the upper and bottom sheet of lap joint.
FIG. 3 illustrates joining of two materials (301, 302) using the refill friction stir spot welding process. As illustrated, there is tendency of a void or cavity (303) formation in the middle of the weld region. An ultrasonic method or x-ray technique is required to detect such defects as the void formed is not visible from exterior of the welded component which if goes undetected may be fatal in certain critical applications as 100% inspection of such defects is not possible. For example, considering the aerospace applications, joining of different parts or segments of different material for the manufacture of aircrafts or aerospace vehicles using refill friction stir welding process may result in unreliable weld joint. Further, the inspection of such thousands of spot joints may be cumbersome task and hence may require more time and cost. Thus, the refill friction stir spot welding still may not be reliable method for spot welding particularly in critical applications such as aerospace. Further, like the conventional friction stir spot welding process, the refill friction stir spot welding process also lacks in joining of two non-weldable dissimilar materials.
Further, the conventional friction stir spot welding processes has limitation of penetrating a filler material into an upper sheet in scenarios wherein the filler material has hardness properties equivalent to that of the upper sheet. More specifically, when the filler material having hardness similar to the upper material is penetrated into the upper material, the plasticization of the filler material takes place and the material is deformed on top of the upper material. Thus, the filler material is not penetrated and hence joining of two materials is difficult in such scenarios. Also, there is no provision in the conventional friction stir spot welding to customize the weld interface properties and thereby enhance the interface material properties.
Thus, in view of this, there is a long-felt need for an improved, novel friction stir spot welding process that addresses the lacunae observed in the conventional friction spot welding and refill friction stir spot welding processes that mainly adheres to limitation of joining non-weldable dissimilar materials. Further, there is a need to facilitate a better provision at the weld region to enable customization of interface material properties.